The invention relates to an electrical connecting cable especially useful for digital systems, comprising a central coaxial unit, around which several coaxial units are wound, each coaxial unit having a central conductor, a dielectric placed around the central conductor and an outer conductor laid around the dielectric; and a sheath serving as an outer cover, made of insulating material.
Connecting cables of this type are intended for connecting electrical circuits in digital systems, in which signals are transmitted from one circuit to the other via the connecting cable.
Examples of digital systems are, for example, computer installations, but especially also telephone exchanges.
The distance to be bridged by a connecting cable varies on the whole from 1 meter to several tens of meters. The total length of cable required for instance in a modern telephone exchange controlled by computers lies in the order of several kilometers. In such cases, the various connecting cables form intricate, bulky cable harnesses which run in cable channels between and along the large number of racks in which the electrical components to be connected, such as printed circuit panels, are accommodated.
It is known to connect the various components by means of single coaxial cables each comprising a central conductor, a dielectric, an outer conductor and an insulating sheath. In this, the central or signal conductor of the coaxial cable is connected with a contact point of a plug which, in its turn, is electrically connnected to, for instance, a panel. The outer conductor or ground conductor of the coaxial cable is connected to an adjacent contact point, also called the ground contact, of the plug. If several signals must be transmitted, then a corresponding number of single coaxial cables is necessary, all being connected to the adjacent contact points of the same plug in the above mentioned way.
The use of separate coaxial cables has various disadvantages. In the first place, the insulating sheath and the dielectric of each coaxial cable must be partly removed, in order to connect the central conductors and the outer conductors to the respective contact points of the connecting plug. Particularly, the connection of the outer conductors to the ground contacts of the plug is laborious, since each outer conductor must be provided with an additional ground connection such as a metal wire soldered to the outer conductor, which is subsequently connected to the contact point of the plug. As a result of self-induction of these ground connections the signalling behavior of the connecting cable is not optimal.
Another serious disadvantage is the unfavorable spatial arrangement of the separate coaxial cables. Often the cables are bundled by means of clamping rings into relatively bulky bundles which lead to practical problems when they are led through the racks of circuit panels.
There are moreover commercially available connecting cables in which several uninsulated coaxial wires are wound around a central coaxial wire and are surrounded by a common insulating sheath. This spatial arrangement is improved in comparison with the aforementioned system of separate coaxial cables. However, the flexibility and also the signalling behavior clearly leaves much to be desired. The outer conductors of the known connecting cable are formed by metal foil or metalized plastic foil, such as a polyester foil which is provided with a layer of aluminum.
It has become apparent that the electrical contact between the outer conductors is not optimal. Thus a resistance of approximately five ohms may occur between the foils. Moreover, the self-induction of the foils is too high, which causes a considerably increased attenuation when high-frequency signals are transmitted. It also has to be borne in mind that only six identical coaxial units, that is, units with the same dimensions and impedance, can be wound around a circular central coaxial unit in one layer, so that the single-layer cable contains a total of seven coaxial units. Many installations and exchanges are controlled by means of binary systems in which the number of connecting lines are powers of two, e.g. 4, 8, 16 etc. In this respect, a connecting cable with seven coaxial units is impractical.